Process oils having the above properties are used in the preparation of rubber tires. For environmental and health reasons it becomes increasingly important to reduce the content of polycyclic aromatics in rubber tire products and therefore also in the process oils which are used for their preparation.
EP-A-950703 describes a process to prepare a process oil as described above. The process oil is prepared by contacting a mixture of a base oil and an aromatic rich extract with furfural as the polar solvent. The base oil is either obtained by hydrorefining or solvent refining. The aromatic rich extract is obtained by solvent extraction of a distillate fraction. This distillate fraction is obtained in a vacuum distillation of the residue of an atmospheric distillation of a crude oil. A disadvantage of this process is that the starting base oil is relatively expensive, when reviewing the numerous solvent or hydrorefining process steps to prepare the base oil.
Another process for preparing such a process oil is known from EP-A-417980. This publication describes a process in which first a petroleum fraction boiling in the lubricating oil range is extracted with a solvent, typically furfural or NMP. The extract mixture obtained is subsequently extracted with the same solvent in a second extraction step in which the process oil is obtained as the raffinate product.
The process of EP-A-417980 is either carried out in two extraction columns or in a so-called blocked-out operation in which the process oil is prepared in one and the same extraction column. In a blocked out operation the feed for the second extraction is first prepared and stored. The extraction column is subsequently prepared to perform the extraction of the stored extract mixture to obtain the process oil. Both schemes are disadvantageous. The first one because it requires an extra extraction column. The second option is disadvantageous because it requires significantly different operating conditions than those used for normal operation. Normal operation being the extraction of waxy distillate (fractions of the vacuum distillation) to make petroleum fractions poor in aromatics which are suitable for subsequent processing to make base oils. It would be desirable to use an existing solvent extraction unit to prepare the process oil. With the process according to this publication however switching between the two operating modes will be time consuming and laborious. Furthermore switching between the two production modes is likely to produce products that are unsuitable for subsequent processing to produce either base oils or the process oils, as described in EP-A-417980, that fall within the specifications of these products. It is therefore likely that these intermediate materials will have to be disposed of in other, less economical refinery streams, for example as the feedstock to a catalytic cracker.
An alternative route to the manufacture of process oils suitable for the application in rubber tires involves the extraction of a petroleum fraction boiling in the lubricating oil range which is extracted with furfural under “medium” severity processing conditions as described in Tire Technology International 1998. The disadvantage of this process route is that the “medium” severity operation differs from those normally used for the production of base oils. Thus when such a medium severity process would be used in an existing solvent extraction process problems will occur with regard to for example the throughput of heat exchangers resulting in clogging and other obvious problems.
U.S. Pat. No. 5,840,175 and U.S. Pat. No. 5,853,569 describe processes to prepare a process oil which can be used as rubber extender oils from naphthenic crude oils. A disadvantage of the processes as disclosed in these publications is that the content of aromatics in the process oil is low while the content of polycyclic aromatic compounds is relatively high. A further disadvantage is that the process comprises both hydrotreating steps and solvent refining steps. The present process aims at providing a process wherein additional hydrotreatment steps are not necessary.